1. Field of the Invention
The present invention relates to new polyoxazolidines, the procedure for their preparation and their use as cross-linking agents for hydrocuring systems based on polyisocyanates, acrylate polymers and polyepoxides in compositions for paints, coatings, sealants and adhesives.
2. Discussion of the Background
U.S. Pat. No. 3,743,626 describes the use of some polyoxazolidines as curing agents in conditions of normal temperature and moisture, for adhesives based on both aromatic and aliphatic polyisocyanates. As described in U.S. Pat. No. 4,138,545, these polyoxazolidines can be obtained by means of a reaction of an oxazolidine (A): ##STR2## with lower alkyl ethers of dicarboxylic or polycarboxylic acids, by transesterification or by means of a reaction of an oxazolidine (B): ##STR3## with a glycol or polyol, under transesterification conditions. The oxazolidines (B) are in turn obtained by adding aldehydes to an addition product between ethanol amine and an alkyl acrylate.
These transesterification reactions are generally carried out in the presence of catalysts and the polyoxazolidines thus obtained have ester groups which can easily undergo hydrolysis.
Belgian patent 856.893 describes the use of some polyoxazolidines in sealant compositions based on polyisocyanates. As described in Belgian patent 833.821, these polyoxazolidines can be obtained by the addition of the oxazolidine (A) to polyisocyanates. These products have the disadvantage of cost and toxicity due to the polyisocyanates. In addition, owing to the formation of urethanes in their synthesis, the viscosity of these products is too high, especially when non-aliphatic diisocyanates or polyisocyanates in general are used.
European Patent Application No.228.935 describes the use of polyoxazolidines as cross-linking agents in sealing compounds based on polyisocyanates, cross-linkable in normal conditions of moisture. These polyoxazolidines use bisalkanolamines (C) as starting products: EQU HO--CH.sub.2 --CH.sub.2 --NH--(CH.sub.2).sub.n --NH--CH.sub.2 --CH.sub.2 --OH
The synthesis of these alkanolamines, starting from amines and ethylene oxide, is not very selective. It is also necessary to separate these reaction products from the reaction mass, under conditions of high temperature and vacuum. Distillation is necessary to eliminate the tertiary amines (tri- and poly-alkanolamines) which, if introduced into the polyisocyanate systems, reduce their life due to early cross-linking, both chemically (alkanols) and catalytically (presence of tertiary nitrogen).
U.S. Pat. No. 4,296,225 describes the use of polyoxazolidines as latent cross-linking systems, in polyvinylic systems, in the preparation of polyvinylic emulsions. In this case, the oxazolidine is used in the form of hydroxyalkyloxazolidine methacrylate, or as a component in polyurethane paints having a high solid content. The principle consists of inserting the oxazolidinic nucleus into a polyacrylate, which is made possible by using a vinyloxazolidine capable of copolymerizing to various degrees with the acrylic monomers. In any case, the oxazolidinic equivalent is not high and the polymers are excessively viscous solids or liquids, making it necessary to disperse them in water or dissolve them in an organic solvent.
These drawbacks of the known art have been brilliantly overcome by the Applicant in Italian Patent Application No. 19089A/89 which describes a group of polyoxazolidines which can be prepared both easily and practically and which can be used as cross-linking agents in hydrocuring systems based on polyisocyanates, acrylate polymers or polyepoxides.
The only disadvantage of this product is the presence of one or more thioether bridges in the structural formula, which although giving stability to photo-oxidation and better oleo-resistance to the manufactures into which these products have been incorporated, generally have an unpleasant odour, typical of polysulphides.